Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. (About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.)
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is, Ag-based epoxidation catalysts that are especially efficient at catalyzing the desired product reaction of ethylene and oxygen to ethylene oxide rather than the side reaction of ethylene and oxygen, which produces carbon dioxide byproduct (and water).
However, while high selectivity catalysts have reduced the formation of carbon dioxide byproduct they may also have increased the production of other undesired byproducts, notably aldehydic impurities such as acetaldehydes and formaldehydes and their associated acids. Acetaldehyde and formaldehyde have long been known as byproducts formed during the operation of ethylene oxide plants. Acetaldehyde is formed as a result of the isomerization of ethylene oxide, while formaldehyde is formed by the reaction of ethylene oxide with oxygen. The associated acids, acetic acid and formic acid, are produced by oxidizing acetaldehyde and formaldehyde, respectively.
While an impurity like carbon dioxide is produced almost exclusively on the catalyst bed in the EO reactor, acetaldehydes, formaldehydes and their associated acids are produced both on the catalyst and past the catalyst bed. Aldehydes and their associated acids can negatively affect the UV quality of the ethylene glycol solution and thereby cause degradation of fiber grade ethylene glycol product. Additionally, the formation of their associated acids (as well as their aldehydic reagents) can decrease the pH to levels sufficiently low to cause corrosion in the plant. These considerations are even more serious in plants that produce Fiber Grade MEG (monoethylene glycol).
One possible method of preventing or reducing the corrosion caused by acidic pH levels is to replace the carbon steel components with stainless steel components. However, this is not only extremely expensive but, at best, it only reduces the rate of corrosion rather than preventing the occurrence of corrosion. Moreover, this of course does not address the problem of low ethylene glycol product quality.
Another possible solution is disclosed in U.S. Pat. No. 4,822,926 in which the reactor product stream is supplied to a quench section (the quench section being disposed inside the EO absorber), and in the quench section the reactor product stream is contacted with a base-containing recirculating aqueous solution in order to neutralize the pH and eliminate some of the organics.
The addition of base like sodium hydroxide does reduce the pH (and as a consequence reduces or eliminates the corrosion in the plant) as well as prevent the formation of some of the organics and aldehydic impurities. But the addition of caustic also frequently causes the decomposition and degradation of the ethylene glycol product this is especially the case for heavier ethylene glycols like triethylene glycol, which often cannot be manufactured to meet minimum quality standards in a process utilizing caustic. Thus, in the end, caustic addition merely exchanges one problem (corrosion and impurity formation) for another (poor product quality).
Far better for eliminating aldehydic and other impurities from the cycle water are ion exchange resins such as those disclosed in U.S. Pat. No. 6,187,973. These ion exchange resins are extremely effective at removing the impurities from the cycle water, without causing the negative consequences mentioned above that result from caustic treatment.
While the use of ion exchange resins is far superior to other techniques, some difficulties with their use still exist. For example, certain organics such as long-chain hydrocarbons can damage the ion exchange resins. One such hydrocarbon species is long-chain esters, which are produced as a result of the build-up of aldehydic impurities and acids in the ethylene oxide as well as the glycol section. These aldehydic impurities then readily react with ethylene oxide and ethylene glycol to make a long-chain ester. For example, formic acid reacts with ethylene glycol to produce ethylene glycol monoformate, which can in turn successively react with more formic acid to produce heavier homologs (i.e., longer-chained hydrocarbons) of ethylene glycol monoformate.
Long-chain esters damage the ion exchange resins because although they are easily adsorbed onto the surface of the ion exchange resin, once adhered to the surface they become elution-resistant, i.e., they do not elute during regeneration meaning that they remain “trapped” on the surface of the ion exchange resin. This reduces the capacity of the ion exchange resin which in turn requires more frequent regeneration cycles. Moreover, the presence of these impurities may also cause resin swelling which can slow the flow of reactants through the ion exchange resin, reducing its throughput.
Hitherto, there has been no technique available for dealing with these long-chain hydrocarbons and the damage they do to ion exchange resins. Instead, the efforts of investigators have focused on techniques for reducing impurities in glycol solutions using ion exchange resins and overlooked that, in the course of performing their function, ion exchange resins often become damaged by continually adsorbing elution-resistant long-chain hydrocarbons. These damaged ion exchange resins are then much less effective at removing impurities and require more frequent regeneration.
Given the foregoing there is a continuing need in the art to reduce the damage to the ion exchange resins caused by these elution-resistant impurities, which become trapped on the surface of the ion exchange resin and do not readily elute during regeneration.